The parallel-axis combination differential gear unit, as shown in FIG. 9, includes a housing (not shown) driven for rotation about a rotational axis L, a pair of side gears S1, S2 rotatably supported within the housing with their axes aligned with each other and coaxial with the rotational axis L, and at least one pair of planetary gears P1, P2 rotatably disposed within the housing in parallel to the rotational axis L. Each of the planetary gears P1, P2 is provided on one end portion thereof with a long gear portion (first gear portion) Pa, on the other end portion thereof with a short gear portion (second gear portion) Pb, and on an intermediate portion thereof with a neck portion Pc having a smaller diameter than the gear portions Pa, Pb. The long gear portion Pa of the planetary gear P1 is in engagement at its inner portion Pa1 with the side gear S1 and at its outer portion Pa2 with the short gear portion Pb of the planetary gear P2. The long gear portion Pa of the planetary gear P2 is in engagement at its inner portion Pa1 with the side gear S2 and at its outer portion Pa2 with the short gear portion Pb of the planetary gear P1 (see WO 93/02301).
In some arrangements of the parallel-axis combination differential gear unit, a plurality of washers, spacers, or the like is arranged between the side gear S1 and the side gear S2. For example, in an arrangement discussed in U.S. Pat. No. 5,389,048, spacer blocks are arranged between a pair of side gears. In such an arrangement, it becomes necessary to enlarge the space between the side gears S1, S2 to the extent of a space occupied by the spacer blocks. To fulfill this requirement, the neck portions Pc of the planetary gears P1, P2 are increased in lengthwise dimension.
However, if the lengthwise dimension of the neck portions Pc of the planetary gears P1, P2 is increased, it gives rise to a problem that the planetary gears P1, P2 are reduced in physical strength. In particular, the neck portion Pc is reduced in torsion strength.